1. Field of the invention
The present invention relates to a method for patterning a layer on an oxide superconductor thin film and a superconducting device manufactured by the method, and more specifically to a method for patterning a layer on an oxide superconductor thin film without degrading the oxide superconductor thin film, and a superconducting device manufactured by the method.
2. Description of Related Art
Devices which utilize superconducting phenomena operate rapidly with low power consumption so that they have higher performance than conventional semiconductor devices. Particularly, by using an oxide superconducting material which has been recently advanced in study, it is possible to produce a superconducting device which operates at relatively high temperature.
Josephson device is one of well-known superconducting devices. However, since Josephson device is a two-terminal device, a logic gate which utilizes Josephson devices becomes complicated. Therefore, three-terminal superconducting devices are more practical.
Typical three-terminal superconducting devices include two types of super-FET (field effect transistor). The first type of the super-FET includes a semiconductor channel, and a superconductor source electrode and a superconductor drain electrode which are formed closely to each other on both side of the semiconductor channel. A portion of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode has a greatly recessed or undercut rear surface so as to have a reduced thickness. In addition, a gate electrode is formed through a gate insulating layer on the portion of the recessed or undercut rear surface of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode.
A superconducting current flows through the semiconductor layer (channel) between the superconductor source electrode and the superconductor drain electrode due to a superconducting proximity effect, and is controlled by an applied gate voltage. This type of the super-FET operates at a higher speed with a low power consumption.
The second type of the super-FET includes a channel of a superconductor formed between a source electrode and a drain electrode, so that a current flowing through the superconducting channel is controlled by a voltage applied to a gate formed above the superconducting channel.
Both of the super-FETs mentioned above are voltage controlled devices which are capable of isolating output signal from input one and of having a well defined gain.
However, since the first type of the super-FET utilizes the superconducting proximity effect, the superconductor source electrode and the superconductor drain electrode have to be positioned within a distance of a few times the coherence length of the superconductor materials of the superconductor source electrode and the superconductor drain electrode. In particular, since an oxide superconductor has a short coherence length, a distance between the superconductor source electrode and the superconductor drain electrode has to be made less than about a few ten nanometers, if the superconductor source electrode and the superconductor drain electrode are formed of the oxide superconductor material. However, it is very difficult to conduct a fine processing such as a fine pattern etching, so as to satisfy the very short separation distance mentioned above.
On the other hand, the super-FET having the superconducting channel has a large current capability, and the fine processing which is required to product the first type of the super-FET is not needed to product this type of super-FET.
In order to obtain a complete ON/OFF operation, both of the superconducting channel and the gate insulating layer should have an extremely thin thickness. For example, the superconducting channel formed of an oxide superconductor material should have a thickness of less than five nanometers and the gate insulating layer should have a thickness more than ten nanometers which is sufficient to prevent a tunnel current.
In the super-FET, since the extremely thin superconducting channel is connected to the relatively thick superconducting source region and the superconducting drain region at their lower portions, the superconducting current flows substantially horizontally through the superconducting channel and substantially vertically in the superconducting source region and the superconducting drain region. Since the oxide superconductor has the largest critical current density J.sub.c in the direction perpendicular to c-axes of its crystal lattices, the superconducting channel is preferably formed of a c-axis oriented oxide superconductor thin film and the superconducting source region and the superconducting drain region are preferably formed of a-axis oriented oxide superconductor thin films.
In a prior art, in order to manufacture the super-FET which has the superconducting channel of c-axis oriented oxide superconductor thin film and the superconducting source region and the superconducting drain region of a-axis oriented oxide superconductor thin films, a c-axis oriented oxide superconductor thin film is formed at first and the c-axis oriented oxide superconductor thin film is etched and removed excluding a portion which will be the superconducting channel. Then, an a-axis oriented oxide superconductor thin film is deposited so as to form the superconducting source region and the superconducting drain region.
In another prior art, at first an a-axis oriented oxide superconductor thin film is deposited and etched so as to form the superconducting source region and the superconducting drain region, and then a c-axis oriented oxide superconductor thin film is deposited so as to form the superconducting channel.
In the above methods, the oxide superconductor thin film is mostly processed by photolithography. Namely, the oxide superconductor thin film is masked by a photoresist and etched by a wet etching process using a weak H.sub.3 PO.sub.4 solution, or a dry etching process such as a reactive ion etching or an ion-milling using Ar ions. In order to process the oxide superconductor thin film without degradation, the oxide superconductor thin film should be prevented from contacting with water. Since the oxide superconductor has high reactivity so as to react with water and is degraded. Therefore, these etching process use little water.
However, an oxide superconductor also reacts with photoresist remover so that a surface of the oxide superconductor thin film on which a photoresist is formed and removed is roughened. It is very difficult to deposit another thin film or layer on the rough surface of the oxide superconductor thin film so as to manufacture a superconducting device or a superconducting circuit of a multi-layer structure. In addition, if another oxide superconductor thin film is formed so as to contact the rough surface, an undesirable Josephson junction or a resistance is generated at the interface. Furthermore, superconducting characteristics of the reacted oxide superconductor thin film is affected, so that the superconducting device does not have an enough performance.